1. FIELD OF INVENTION:
The present invention relates a sound source for generating acoustic pulses in a well borehole.
2. DESCRIPTION OF PRIOR ART:
It has been desirable to be able to perform seismic exploration of subsurface formations around a well borehole and beneath the borehole. Conventional seismic exploration of the formations surrounding and below boreholes was done using vertical seismic profiling (VSP) techniques. Typically, shots were fired in shallow boreholes, and seismic waves recorded by a detector lowered into the deep borehole on a logging cable and positioned at successive depths. One shot was fired for each depth. One trace was recorded representing the direct arrivals and reflections that could be observed by a detector at the surface from each shot. If multiple offsets were desired, several shots had to be fired at different radial distances from the borehole, and this procedure repeated as the detector was moved to different depths in the borehoe. A complete multiple offset VSP survey of a borehole was a very time-consuming operation and thus was not done very often because of the expense involved.
It has long been known that in theory the shots could be detonated in the deep borehoe, and multiple detectors could be located at the surface to record all the offsets of a multiple offset VSP simultaneously. This procedure has been named "Reversed VSP." The reason this has not become an operational technique is the difficulty of getting a suitable deep hole sound source. Explosives might be used; however, only a small number of shots can be lowered in the hole at one time. A complete survey involves many shots and would require many trips into the hole using conventional explosive devices such as perforating gun technology. Additionally, due to the limitation on the size of charge that can be detonated in the borehole, several shots would probably be required at each depth. This would further multiply the number of trips into the hole.
Acoustic pulses could be generated by piezoelectric crystals and other devices lowered into the borehole by a well logging cable. The cable could supply the devices with continuous electrical power and avoid the problem of making multiple trips into and out of the hole. The principal problem with this technique is that the amount of power that could be transmitted through a logging cable would be low, typically about 200 to 300 watts. The pulses generated would thus be much lower in energy than those generated by explosives; or, alternatively very long time periods would be required to store up enough energy in the downhole tool to discharge into a high-energy pulse. In either case, very long periods of time would be required at each depth in the borehole to transmit sufficient energy to produce acceptable signal-to-noise ratios at the detectors on the surface. Examples of such a seismic source receiving and storing operating power from the surface via a conduit are U.S. Pat. No. 3,979,140 and U.S. Pat. No. 4,682,309.
There have been attempts in the prior art to form acoustic signals in well boreholes. For example, in U.S. Pat. No. 2,898,084 a drill string was lifted so that a rotary drill bit was raised from of the well bottom. During such lifting, telescoping inner and outer members armed a seismic shock source. When lifting force was released, surfaces on the inner and outer members transferred the weight of the drill string to the drill shortly after the drill bit had hit the well bore bottom. A disadvantage with this was that the rotary drill bit was being used as a hammer or ram against the well bottom. Other hammer and anvil techniques were disclosed in U.S. Pat. No. 4,569,412 and U.K. Pat. No. 2,147,700B.
Another method, based on the "water hammer" effect, was to temporarily block the flow of drilling mud, then release flow. The surge in flow caused an acoustic pulse. Examples of this technique were disclosed in U.S. Pat. Nos. 3,993,974 and 4,252,209.